As far as not picking up the router outside the room goes, your house should not be an issue as I am using a USR wireles router/USB dongle/whatever is in laptops and I have not had issues with getting a signal through a reinforced concrete floor (house is similar, one PC on "ground" floor so signal has to go through floor AND solid brick wall, mainPC and router in lounge on first floor, laptop can be anywhere but is usually top floor. No extenders) so I wonder about the transmit power settings to start with..
I'd really have to see the setup to figure it out, but it's strange it happened after changing dsl modem and I can't figure out why that would be an issue. Do you still have the old modem to try your with your setup and what is the make/model of the new modem?